On-going geo-political concerns, concerns for global warming, and rising oil prices are fueling the push for renewable energy sources such as wind and solar power. Today, the majority of the electricity generated in the United States is produced by burning fossil fuels, such as coal, natural gas, and petroleum, nuclear power and hydroelectric power. Energy produced from alternative energy sources, such as wind and solar power, account for a small percentage of the total electricity produced in the United States. Our reliance on fossil fuels and nuclear power has several drawbacks. While fossil fuels are comparatively inexpensive, there is only a limited supply of fossil fuels, which will eventually be depleted if alternative energy sources are not found. Further, the burning of fossil fuels to produce electricity emits greenhouse gases that contribute to global warming. Nuclear power presents environmental and nuclear proliferation hazards.
Solar energy and wind power are promising alternative energy sources that can reduce reliance on fossil fuels for generating electricity. Solar energy and wind power are renewable resources so there is no concern about future depletion of these resources. Further, the generation of electricity from solar energy and wind power does not emit greenhouse gases and is therefore considered more environmentally friendly. Also, generation of electricity from renewable energy sources does not generate hazardous by-products that need to be disposed of.
In the field of solar energy utilization, much work has been done to develop a system that is economical enough to replace the combustion of fossil fuels to supply the growing needs of our ever more highly populated planet in a clean and renewable way. Solar photovoltaic systems have received the most attention over the years and yet suffer from a very high cost of equipment. Even with government subsidies, 25 year payback periods are common. In addition, because electricity is very expensive to store, the use of periodic energy sources such as solar to produce electric power is problematic.
A better approach would be to use solar energy to produce a fuel, which could be stored and transported easily and economically. This fact has led to a great deal of research into ways to use sunlight to produce fuels from low energy feedstocks, most notably to produce hydrogen from water. This arises largely from the well-known fact that hydrogen, when used as a fuel produces only water, which truly makes hydrogen a “clean fuel” candidate. Despite the large body of work on this topic there is not yet an economical process in practice.
Electricity for electrolysis may come from renewable resources such as solar or wind, but ˜60 kWhr is required to produce 1 kg of hydrogen from water electrolytically. This limitation renders this process too expensive to compete in the energy marketplace with fossil fuels.
Direct thermal decomposition of water has been proposed as a possible way to avoid the inefficiencies and expense of the photon-to-electron conversion step that limits the aforementioned solar electrolysis system approach. The follow formula illustrates the decomposition:2H2O+Heat→2H2+O2 To generate thermal decomposition of water, very high temperatures are required to produce appreciable amounts of reaction products, i.e., hydrogen and oxygen. For example, at equilibrium, 2% of water decomposes at a temperature of 1800° C. and at a pressure of 760 Torr. This high temperature requirement imposes very strict requirements on reactor materials, because they are exposed to very high temperatures and very reactive gasses. Thermal shock and very large thermal gradients are also a concern because mismatches in thermal expansion coefficients among different construction materials can cause cracking and mechanical failure. Besides thermal and chemical stability, the direct thermal decomposition system should also be able to allow for the separation of the reaction products, oxygen and hydrogen from each other to avoid recombination.
In order to accomplish this separation, the gases must either be cooled rapidly and then separated later or separated when hot. Rapid cooling or quenching, while potentially effective at preventing recombination, also limits the process in 3 important ways. First, it limits how far the decomposition reaction may proceed to what is formed at equilibrium for a given temperature and pressure. Second, it inevitably results in significant heat loss during quenching as most of the water vapor remains unreacted and all of that unreacted water vapor must be cooled along with the product gases. Third, while hydrogen and oxygen may not spontaneously recombine while cool, this approach does present a gas mixture that can be very dangerous to work with.
Based on the above, separating the hydrogen and oxygen gases from each other while still at decomposition temperatures may be an attractive approach. This separation can be accomplished by using ceramic-based high temperature hydrogen permeable membranes and/or high temperature oxygen permeable membranes. As a result, work in this area has taken place, however successful implementation of a commercially successful system with sufficient robustness and gas throughput has not occurred.
Previously, two types of approaches have been attempted. In the first type, the separation of both products at or near decomposition temperatures has been attempted. This approach ultimately failed because, at the very high temperatures required, commercially viable (existing) hydrogen separation systems were unsuitable. In the second type, only oxygen was removed at decomposition temperatures, leaving hydrogen and water vapor to be separated later by a phase change, i.e., by cooling and condensing the water. The disadvantage of this type of approach is that, like the rapid quenching method, energy loss is excessive.
As described above, a dearth of suitable materials along with the challenges of energy efficiency associated with such a high temperature process has kept reaction (1) from becoming a commercially viable process for solar energy utilization. Indeed, the number of known materials that can function at such a temperature and in the required chemical environments is quite small. For example, graphite and silicon carbide possess exceedingly high melting points, but would themselves react upon exposure to hot oxygen according to the following reactions:C+O2→CO2+Heat  (2)SiC+2O2→SiO2+CO2+Heat  (3)
Previous work has suggested that actively removing oxygen and hydrogen could drive reaction (1) to favor product formation thereby allowing for lower operating temperatures. While this is true, at temperatures where both hydrogen and oxygen permeable membranes are stable, the concentrations of products formed are typically too small. In order to create the gradient required to drive the separation of oxygen (e.g., at commercially relevant rates), hydrogen or hydrocarbons have to be consumed catalytically on the permeate side of the oxygen membrane. As a result, this approach is commercially self-defeating. In addition, schemes that separate only the oxygen and later use condensation of the remaining water vapor in order to recover hydrogen suffer from significant energy inefficiencies.